I have two RJ45 sockets connected by an inaccessible unknown-category twisted pair cable. I plug two unmanaged gigabit switches into the sockets using Cat6 cables, and they negotiate a 100Mbps link. I suppose there's nothing I can do to try and get them to connect at 1Gbps?

edit Connected back-to-back, the switches negotiate 1Gbps.

edit The sockets are labelled Cat5e, which gives some indication of the age of installation. The cable has four pairs and all appear to be wired at both ends. Is there any way to test the wiring on the cheap? This thing looks like it might do the trick -- any opinions?

edit The cheap continuity tester helped diagnose and fix the problem in no time.

3 Answers
3

If they're not completely inaccessible, you could pull the cover off one of the jacks. If only two pairs are wired, there's part of your problem. If all four pairs are available, reterminate each end using new jacks and all four pairs. Cross your fingers. It still might not work.

Wonderful, thanks! This is precisely the sort of stuff I am looking for.
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NPEDec 12 '10 at 14:40

I've had a look, and all four pairs are wired (the suggestion still gets (+1) from me). Are there any tricks of the trade for testing the electrical connection between the 8 pairs of wire ends on the cheap? The sockets are about 5m apart. Thanks!
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NPEDec 12 '10 at 14:49

@aix: Short of buying, renting or borrowing a cable analyzer that can actually certify the run (rather than just doing simply continuity tests), there's not much you can do. Reterminating gives you a small chance of fixing it if there was poor termination. Well-installed Cat 5e is gigabit-capable, but there are lots of opportunities to foul that up.
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Dennis WilliamsonDec 12 '10 at 16:26

5M isn't much cable. How about using the old cable to pull in new cable? The only potential issue I could see would be if the the installer used staples on the old cable, which would also likely explain why your speed stinks.

In case any holes drilled are only big enough for a single cable, you might want to attach a string to the old cable, pull that through, then use the string to pull through a new cable.

The type of test unit to which you refer will help you determine if the cable is wired in the correct order and is continuous from end-to-end, but it won't test the quality, such as problems crimps or staples will cause.

If they're unmanaged, nope. Try connecting them back-to-back with a known short length of high-quality CAT6 cable. See if you get GigE speeds then.

Sorry, this is why you don't buy shitty switches. You get shitty performance, and no management features. That said, even if you did override the settings, you'd still get crap performance, and probably lose packets. Chances are, you're going over ancient structured cabling that's been there for YEARS.

I've already tried this, and a back-to-back connection with a single high-quality cable does give gigabit speeds. This is a home network, so high-end kit is out of the question on the basis of cost. Besides, I suspect the problem is the (inaccessible) cable rather than the switches.
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NPEDec 12 '10 at 12:13

I think you've basically answered your own question and proved it quite well. If you've tested with short/good cable and it works, and with long/old cable, and it doesn't. Ergo, the problem is the cable.
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Tom O'ConnorDec 12 '10 at 12:24

With respect, that was never the question. The question was -- if you re-read it -- is there a way to force the connection at 1Gbps (basically to see how well that works, if at all)? I expect the answer to be "no", but I am posting here in case somebody has something different to say.
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NPEDec 12 '10 at 12:33